(1) Field of the Invention
This invention relates in general to an overhead valve type internal combustion engine and, in more particular, to the intake and exhaust system arrangement for such engines.
(2) Description of the Prior Art
Overhead valve type internal combustion engines characterized by their arrangements that the intake and exhaust valves are disposed in the top portion of the combustion chamber, are universally applied in rolling stock and other work vehicles for agriculture and public engineering.
Work vehicles normally employ general-purpose engines, which can be broadly classified into two groups depending on the disposition of the crankshaft. In the first group, the crankshaft is vertically suported with horizontally placed cylinders. Those belonging to the other group have their crankshafts horizontally arranged, with the cylinders being vertically held.
For application particularly in lawn mowers and mowing machines, vertical crankshaft types having their output shaft provided at a lower end thereof are more popular due to less overall engine height and the greater stability of the equipped machine.
In the cylinder heads of those conventional overhead valve type internal combustion engines, the inlet ports, which are connected to the carburetor and air cleaner, are bored at locations opposite the corresponding exhaust ports to which the exhaust duct and muffler are joined for exhaust gas emission.
A fan is generally mounted at the remotest end of the crankshaft from the output shaft, which is provided for cooling the cylinders and cylinderhead by supplying cooling air to them through an air duct.
A representative such engine is disclosed in U.S. Pat. No. 4,570,584.
However, those prior art overhead valve type engines have been found to pose problems. First, because of their designs that the inlet port is located opposite the exhaust port in the cylinder head, the bores themselves and their portions enclosing the push rods stand in the way of air streams from the cooling fan, so that no sufficient cooling of the cylinder head cannot be achieved.
Moreover, since insufficient cooling of a cylinder head tends to result in abnormal combustion problems such as knocking, the engine cannot enhance output and insure high fuel economy, one of the expected advantages of the overhead valve type internal combustion engine.
The valve guide is normally pressure fitted into the exhaust or intake valve bore and is usually made of iron since it is more likely to pose heat problems. In overhead valve type engines, the exhaust valve guides are normally mounted in the cylinder head on the crankshaft output side. Another problem with them is that great difficulty is encountered in cooling the exhaust valve guides in particular by a cooling fan. As result, exhaust valve guides, exposed to the high temperatures of exhaust gases, tend to slip out of places in the cylinder head, which are normally made of cast aluminum.